Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 54)

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54 Rev. 1.3
Figure 22. Modem Module V
CC
and RESET Filter
4.5. Analog Output
The call progress tone provided by AOUT and discussed in this section comes from a PWM output pin on the
ISOmodem. AOUT is a 50% duty cycle, 32 kHz square wave, pulse-width modulated (PWM) by voice band audio,
such as call progress tones.
The PWM signal should be processed by a high-pass filter (R2, R3, R4, C2,C3 and C4), and, with the aid of a
bridge mode amplifier, provides low-cost 100 mW to 250 mW power with a 3 to 5 V supply. See the circuit in
Figure 23. A slightly more expensive amplifier (LM4862) is available and, while still pin-compatible, provides twice
as much power.
Figure 23. PWM Audio Processing and Amplifying Circuit
4.5.1. Interaction between the AOUT Circuit and the Required Modem Reset Time
When modifying the circuit shown in Figure 23, it is important to examine the reset timing and know that when
external reset is applied to the modem, the AOUT pin still has time to rise to VCC due to the pullup installed on it.
One has to assume that the modem has been operating prior to reset and has put AOUT into a PWM state that is
100% low.
This is important because the AOUT pin, which is shared with INT in some packages, is read by the strapping
option logic in the modem at the end of the reset time to set the operational mode as shown in "2.1.3. Reset-Strap
Options for 16-Pin SOIC Package" and "2.1.4. Reset-Strap Options for 24-Pin TSSOP Package" on page 13 and
"2.1.5. Reset Strapping Options for QFN Parts" on page 15.
The value of the capacitors and resistors in the above circuit thus has an effect on the minimum required
ISOmodem reset time.
RESET
Motherboard
Connector
1.0 F
10 k 
2.2 F
To RESET
(Si24xx Pin 12)
GND
To Modem Chip V
CC
(Si24xx Pins 5, 21)
GND
VCC
Murata BLM 18A
G601 SN1
0.1
F 0.1 F 1.0 F